Investigation of mechanical, physical and thermoacoustic properties of a novel light-weight dense wall panels made of bamboo Phyllostachys Bambusides

This study was conducted to evaluate the properties of lightweight sandwich panels made from low diameter bamboo particles, Phyllostachys Bambusides collected from Gilan province, Iran, as core layer, combined with thin wall bamboo strips as faces. The effects of three individual variables such as density of core layer (350–550 kg/m3), resin consumption in core layer (7.5–9.5%) and resin consumption in faces (175–275 g/m2) on some important physical, mechanical and thermos-acoustic properties of the panels were investigated. Response surface methodology was used to statistically analyse the results and optimization process. The average values for the mechanical properties of the sandwich panels were obtained as 17.16 MPa, 5669 MPa, 0.02 MPa, 17.60 MPa, 1.83 MPa, 0.03 MPa, and 913.3 MPA for modulus of rupture, modulus of elasticity, internal bonding, compression strength parallel to face grain, compression strength perpendicular to face grain, shear strength, and screw holding, respectively. Finally, thermal conductivity and noise reduction coefficient of the panels were respectively gained as 0.01 W/mk and 0.31. The results of technical and thermo- acoustic properties of the panels showed that the light weight sandwich panels from bamboo residues would be a suitable and sustainable alternative as an insulation material for sustainable and green construction.

the numerical irregularities associated with them (such as CFD or finite element analysis).In RSM, convergence towards the optimal element is preferred because they reduce the effects of disorder factors.In this design, 3 variables (density, core layer resin, skin resin) were considered in 5 levels, and as a result, 15 laboratory panels were produced.
To follow the partial factorial design, 15 laboratory lightweight sandwich panels (density of 600 ± 60 kg m 3 ) were manufactured (Table 2).

Strip preparation and treatment
Strip preparation has been carried out in Wood-based material laboratory, Department of Wood and Paper Science at University of Tehran.Strips were randomly extracted from the length of bamboo culms.This method was chosen to reduce the effect of uncontrollable factors in the final results of the tests, because the presence of nodes along the strip and the extraction location of the strips were not among the variable factors of this research.Bamboo stems (Fig. 1a) were cut into the pieces with around 50 cm were longitudinally cut into the suitable strips (Fig. 1b).For steam treatment, the strips were placed inside a laboratory cooking cylinder (Fig. 1c).Cooking temperature and steaming time inside the cylinder were 150 °C and 1 h, respectively.The pressing operation was performed immediately on the treated strips, in such a way that the strips were placed under a heat press with a temperature of 120 °C and a pressure of 20 kg/cm 2 (Fig. 1d).The flattened strips were placed under heavy steel plates for 24 h and prepared for use in the assembly stage (Fig. 1e).To control the moisture content of the strips, they were kept in a conditioning chamber at 20 ± 3 °C and 65 ± 1 percent RH with constant temperature and humidity for a week.

Flake preparation
Bamboo flakes were used to make the middle layer of the sandwich panels.After delivering low quality bamboo strips and scraps to the laboratory, the bamboo pieces were cut into the smaller ones(8 < x < 15) (Fig. 2a).Knife length, using a band saw.The short stems ring flaker (Pallmann, Germany) was used to convert the bamboo chips into the suitable flaker, in order to obtain the appropriate raw material for making the middle layer of the sandwich panels (Fig. 2b).The bamboo flake was placed in a laboratory tray dryer at 105 °C for 24 h to achieve a moisture content around 2%, then passed through a laboratory flat vibratory screen to separate acceptable sizes for 40 ≤ x ≤ 80 mm.Dried and screened flakes were stored in sealed plastic bags before manufacturing the core panel.www.nature.com/scientificreports/

Light weight panel manufacturing
The schematic view of panel manufacturing is shown in Fig. 3.The panel specifications and parameters for making lightweight sandwich wall panel are presented in Table 3. Diluted adhesive was sprayed onto bamboo particles with coherent parameters using a laboratory rotational drum mixer having an internal spray nozzle.A mixture of bamboo chips and MDI resin was taken out of the gluing machine and manually formed into a 45 × 45 × 2.5 cm frame.At this stage, Burkle laboratory hot press was used.The press temperature was adjusted to 180 °C and a pressure of 24 kg/cm 2 was applied for 8 min.Polyurethane (PUR) adhesive used for bamboo strips in outer layers.The press was set at a pressure of 12 kg/cm 2 , at a temperature of 100 °C, and the panels were pressed for 45 min.Due to the usage of polyurethane resin that does not require heat, we could use no heat for the final assemble, but to speed up the production of the samples, it was decided to use heat during the pressing (Fig. 4).

Panel characterization
All treated panels were stored in a conditioning chamber for two weeks after cold stacking, at 20 ± 3 °C and 65 ± 1 percent RH, in line with (ASTM D1037 32 ) until the panels acquired the standard moisture equilibrium content.

Panel characterization
Physical and mechanical properties Physical and mechanical tests along with thermos-acoustic properties were measured in accordance with DIN EN, ISO and ASTM standards (Table 4).Several standards were used to perform the tests for two reasons.Firstly, it was due to the limitation in the dimensions and number of samples, and the second reason was the availability of equipment in the laboratory, each of which was designed for a specific standard.
Modulus of rupture, Internal bonding, screw holding and compression-shear tests were measured using universal testing machine (Wolpert, Germany), and modulus of elasticity, modulus of rupture and compression  www.nature.com/scientificreports/strength parallel and perpendicular to the face grain were measured using universal testing machine (Instron 4486) (Fig. 5a-e).

Thermo-accoustical testing
Acoustic absorption coefficients and noise reduction coefficient (NRC) were used to determine acoustical properties of the panels.In this study, measurements were made in accordance with ASTM C423 Standard Method (ASTM C423 34 ) using an impedance tube with two diameters as 99 mm and 29 mm (Fig. 6).

Numerical optimization
By numerical optimization in engineering sciences many phenomena according to their own instructions and conditions.However, a number of phenomena do not have the ability to have a suitable mathematical model due to the large number of controlling factors, unknown mechanism or computational complexity.In such cases, the use of experimental modeling methods works.The Response Surface Methodology (RSM) is considered as one of the experimental modeling methods 35 .RSM was originally developed to model experimental responses and then to model numerical experiments.In physical experiments, experimental errors can occur in a variety of ways, for example, estimating errors when the disorder or error is due to a wrong convergence (e.g., the operator is drowsy or tired or the test materials are heterogeneous) or to define a continuous physical phenomenon in a discrete way if such a thing cannot be done in reality.In RSM, the errors are assumed to be random.The application of RSM to design optimization is to reduce the cost of analytics methods and their associated numerical irregularities (such as CFD or finite element analysis).Also, convergence towards the element is optimal because they reduce the effects of disorder factors.

Ethics statement
Four-year-old bamboo (Phyllostachys Bambusides) stems were used in this study.supplied from Lialestan bamboo plant, Gilan, Iran.The collection of the bamboo samples was carried out according to the Convention on Biological Diversity.

Physical properties
Density test was used to measure the density of the samples.The average density of all samples was 594.5 kg/m 3 .
The fabricated sample has a low density and according to EN316 36 standard test method for fiberboard classification, this panel is in the category of light panels and helps to reduce the overall weight of the building significantly compared to the available materials.Obviously, the density factor of the middle layer affects the overall density of the samples, but the other two factors will not affect this value.

Mechanical properties
Mechanical properties of the panels have been shown in Table 6.According to the EN310 standard test method, the bending strength standard for application in OSB boards with a thickness of 18-25 mm and for general use is equal to 16 MPa.This value is equal to 19.6 MPa for particleboard with a thickness of 25 mm in general use and dry conditions, and equal to 33 MPa for MDF with a thickness of 15 mm.The average modulus of rupture for the samples tested in this study is 19.44 MPa.Considering a model consist of three parameters (A = density, B = core resin and C = face resin), analysis of variance demonstrates a siginificant model (Table 5).The number 4.71 for the F-value indicates the significance of the model, and based on the analaysis of variance only 3.64% error is likely to occur.If the P-value is less than 0.05, the model will be significant.The modulus of rupture can be calculated based on the linear relationship described here (Eq.1).Considering the construction of the samples, which consist of a particleboard core and two layers of bamboo strips on both sides, it is possible to analyze their impact on bending loads.The middle board breaks due to the applied load (Fig. 8), and by looking at the failure mode, which is diagonal, it can be concluded that the applied compressive force is distributed as a point along the length of the specimen and a shear failure caused the middle board to break.Due to the construction of the core, which is made of bamboo particles impegranted with Polyurethane adhesive, the amount of force applied which caused a fracture in the middle layer is not high, and the modulus of rupture of the specimens can be considered acceptable.The average modulus of elasticity for the constructed specimens is 5659 MPa.As well as MOR, analysis of variance illustrates a significant model for modulus of elasticity.The F-value of 4.53 indicates the significance of the model and only the possibility for an error is 3.98%.The relationship obtained from the model will be significant and the modulus of elasticity can be calculated using (Eq.2).
The core has low flexibility, and due to the strength of the strips on the outer surfaces, the resistance of the boards to deformation is very good, which indicates the appropriate modulus of elasticity of these boards (Fig. 9).The results of internal bonding measurements are between 0.01 and 0.08 MPa, and the average is 0.02 MPa.The number 2.72 indicates the significance of the model and 9.55% is likely to encounter an error.The relationship obtained from the model is not significant.
The results of compression strength parallel to face grain measurements showed that this value for the samples is between 10.06 and 26.26 MPa, the average of all the samples tested is 17.6 MPa.The low density of the middle www.nature.com/scientificreports/board and its porosity, as well as the amount of adhesive used on the outer surface, have created relatively little adhesion between the strips and the middle board, which disintegrated when the load was applied (Fig. 10).This value will increase as the adhesion improves because the resistance of bamboo strips alone to the parallel compressive force of the fibers is high.This resistance is created due to the structure of bamboo, which is in the form of parallel fibers along the length of its stem.The results of measuring the compression strength perpendicular to face grain showed that this value for the samples is between 1.04 and 11.34 MPa, and the average for all the samples is 4.62 MPa.The analysis of variance in this model shows that it is significant because the F-value is 2.79 and there is only 11.26% possibility for an error (Table 6).
The results of shear test measurements showed that this value for the samples is between 0.01 and 0.532 MPa, the average of all of them is 0.07 MPa (Table 6).
The effect of any of the variable factors on the amount of Compression-shear strength is not significant and based on these variables, the amount of change in this resistance will not be significant (Table 7).The results of     possible to cut a disc with a diameter of 29 mm for this test and therefore it was measured only at low frequencies.The average value for this test was obtained from the frequencies of 250, 500 and 1000 which is equal to 0.4633.The average density of the samples is 594.5 kg/m 3 and the average thermal conductivity is 0.0199 W/mK.The results of the tests indicate that the product is desirable in terms of sound absorption and thermal conductivity (Tables 8, 9).The average MOR and MOE values obtained from bamboo panels are comparable to other materials as shown in Table 8. High percentage of porosity and low density of boards are effective in increasing the coefficients and comparison of samples with similar products shows the effect of the middle board on both coefficients.It should be noted that the presence of bamboo strips on the two outer surfaces of the boards has an effect on reducing the sound absorption coefficient and has caused the sound to be reflected from the surface of the sample.Due to the use of bamboo and fabrication of low density samples, the thermal conductivity is also very desirable according to the test results (Table 9).These new structures are important in several ways due to their lightness and high insulation properties.The low thickness of this composite (2.5 cm) and at the same time its multiple function is the first point.In addition to its separating function and beauty, this structure is also a very good insulator.Another point is that these structures are more sustainable than other structures.If common materials are used, add-ons should be used on the wall to insulate it, however, these panels are also insulated by themselves.This saves energy and is more sustainable.

Optimization
To select the optimal treatment, the Design Expert software optimization plugin was used.The treatment was selected based on the significant results of the tests to increase the desirability.In the analysis for optimizing, only the significant results were considered, these tests are: modulus of rupture, modulus of elasticity, compressive strength perpendicular to the face grains and screw withdrawal strength.Variable factors that were: middle layer density, core layer resin and outer surface adhesive were considered in their specific range.The selected treatment has a desirability of 1. Tables 10 and 11 show the complete specifications of the optimal treatment.

Conclusion
To conclude, in this paper a series of physical, mechanical and insulating performance of a novel light bamboo sandwich panel (LTBSP) as a sustainable green composite (manily based on waste and renewable resources) from bamboo strips and shavings was investigated.
• LTBSP stood a better chance to be applied as a wall panel-according to test results that mentioned before- rather than MDF or knauf wall boards.• Some of the mechanical properties of LTBSP such as modulus of rupture, modulus of elasticity and resistance to axial withdrawal of screws reached the requirements of DIN-EN 312-320 standard test methods.• The thermal conductivity and noise reduction coefficients for LTBSP met the requirements of IS3129-1985 for particleboards.• In view of test results obtained in this study and in comparison to other composites, LTBSP is a suitable mate- rial as a wall panel, considering its components that are environment-friendly, it can be seen as an alternative to synthetic-based commercial products.

Figure 3 .
Figure 3.The panel production process in schematic form.

Figure 4 .
Figure 4. Three layers (core-two outer layers) of sandwich panel under the pressure.

Figure 8 .Figure 9 .
Figure 8. Failure of the core layer due to the compression load.

Figure 10 .Table 6 .
Figure 10.Sample failure modes in compression test parallel to the face grain.

Table 1 .
Specified levels of variables.

Table 2 .
Design of experiments table.

Table 3 .
Constant processing parameters in the production of panels.

Table 4 .
Standard and dimension of the test specimen.

Table 5 .
Average values of the parameters for the panels.

Table 9 .
Thermal conductivity coefficient for different materials.